The present invention relates to a low-profile TEM mode (dominant mode) quarter wavelength (xcex/4) dielectric resonator having a high unloaded quality factor compared to a conventional dielectric resonator, and to a two-pole bandpass filter using this low-profile TEM mode dielectric resonator.
In the two-pole bandpass filter according to the present invention, the coupling between two adjacent resonators is provided by evanescent mode waveguide.
A resonator according to the present invention is expected to be used in a filter, a voltage controlled oscillator (VCO) and an antenna for mobile communication. A filter of the present invention can be used in a cellular phone system such as wide band CDMA (Code Division Multiple Access), and another communication system where filtering is required.
The followings are known literatures:
[1] Arun Chandra Kundu and Ikuo Awai, xe2x80x9cLow-Profile Dual Mode BPF Using Square Dielectric Disk Resonator,xe2x80x9d Proceedings of the 1997 Chugoku-region Autumn Joint Conference of Electric/Information Associated Congress, Hiroshima, Japan, pp. 272 (October, 1997).
[2] Arun Chandra Kundu and Ikuo Awai, xe2x80x9cDistributed Coupling in a Circular Dielectric Disk Resonator and its Application to a Square Dielectric Disk Resonator to Fabricate a Low-Profile Dual Mode BPFxe2x80x9d. 1998 IEEE MTT-S Digest, pp. 837-840, June 1998, Maryland, USA
[3] Yoshihiro Konishi, xe2x80x9cNovel Dielectric Waveguide Componentsxe2x80x94Microwave Application of New Ceramic Materials,xe2x80x9d IEEE Proc., Vol. 79, No. 6, pp. 726-740, June, 1991.
In the literatures [1] and [2], Arun Chandra Kundu who is one of inventors of the present application has proposed a new type TEM dual-mode dielectric disk resonator having the following configuration, and a bandpass filter (BPF) using the resonator.
This dielectric resonator is a dual mode resonator having a square planer shape in 5 mmxc3x975 mm, and its top and bottom surfaces are covered with silver. The top silver layer is floating, and the bottom silver layer is grounded. The interior of the two silver layers are filled with dielectric material of a relative permittivity or relative dielectric constant of 93. All of the side walls of the disk resonator are open surfaces exposed to the air. Accordingly, radiation easily occurs with leakage of electromagnetic field through these open surfaces. An electric field becomes at the maximum on each open surface, and becomes at the minimum along each symmetry plane of the resonator. Therefore this kind of resonator is called a half wavelength (xcex/2) dielectric disk resonator.
FIG. 1 illustrates the result of a theoretically and experimentally verifying relationship between the thickness and the unloaded quality factor Q0 regarding this disk resonator, and a similar graph is described in the literature [1]. As apparent from the figure, the unloaded quality factor Q0 becomes at the maximum (≈250 (experimental value)) when the thickness is 1 mm and the length and the width of the resonator is 5 mmxc3x975 mm using dielectric material with a relative dielectric constant of 93.
Recent mobile terminals demand super compact bandpass filter, and hence it is required to promote further low profiling and compacting of dielectric resonators used inside the portable terminals. However, it is very difficult except that material having a higher dielectric constant is used in order to further miniaturize the dielectric resonator with keeping high performance.
In addition, if a 2 GHz bandpass filter is formed with using the conventional resonator described in the literature [2], the size of the filter become 8.5 mmxc3x978.5 mmxc3x971.0 mm, and its unloaded quality factor becomes 260. The recent mobile terminals, however, demand more compact and higher-performance filters.
It is therefore an object of the present invention to provide a TEM mode dielectric resonator having a minimized size without changing a resonant frequency and an unloaded quality factor.
Another object of the present invention is to provide a bandpass filter using a TEM mode dielectric resonator, whereby the size can be minimized with keeping the performance of the filter.
According to the present invention, a TEM mode xcex/4 dielectric resonator includes a rectangular dielectric block having a top planar surface, a bottom planar surface and four side surfaces, a first metal layer coated on the top planar surface, a second metal layer coated on the bottom planar surface, and a third metal layer coated on one of the four side surfaces.
FIG. 2 illustrates the configuration of a conventional xcex/2 dielectric resonator, and FIG. 3 illustrates the fundamental configuration of a xcex/4 dielectric resonator according to the present invention.
In FIG. 2, reference numeral 20 denotes a dielectric block with a rectangular planar shape, 21 a silver layer coated on a top surface of the dielectric block 20, and 22 a silver layer coated on a bottom surface of the dielectric block 20. The top silver layer 21 is floating, and the bottom silver layer 22 is grounded. All of the four sidewalls of the dielectric block 20 are open to the air. In FIG. 2, the length and width of the xcex/2 dielectric resonator is denoted by xe2x80x9caxe2x80x9d and its thickness is denoted by xe2x80x9ctxe2x80x9d.
Supposing that the TEM mode propagating along z-axis direction in this xcex/2 dielectric resonator, the negative maximum electrical field exists on a plane at Z=0 and the positive maximum electrical field on a plane at z=a, as shown by arrows 23 in FIG. 2. The minimum (zero) electrical field obviously exists on a plane 24 at z=a/2 that is the symmetry plane of the xcex/2 resonator.
It is possible to obtain two xcex/4 dielectric resonators by dividing such xcex/2 dielectric resonator. along this symmetry plane 24 and providing conductors on the divided surfaces.
FIG. 3 illustrates a xcex/4 dielectric resonator formed in this manner. In the figure, reference numeral 30 denotes a dielectric block with a rectangular parallelepiped shape, 31 a silver layer coated on a top surface of the dielectric block 30, and 32 a silver layer coated on a bottom surface of the dielectric block 30. The top silver layer 31 is floating, and the bottom silver layer 32 is grounded. One of side walls of the dielectric block 30 is a shorted end surface of a silver-coated layer 34 for shorting the top and bottom silver layers 31 and 32, and other three side walls are open to the air. In FIG. 3, also, arrows 33 denote a direction of an electrical field, and arrows 35 a direction of current.
The xcex/4 dielectric resonator shown in FIG. 3 and the xcex/2 dielectric resonator shown in FIG. 2 have the same resonant frequency in principle. Due to a high relative dielectric constant of 93, electromagnetic field confinement property is strong enough. Thus, the electromagnetic field distribution of the xcex/4 resonator and xcex/2 resonator is almost the same. As shown in FIGS. 2 and 3, the volume of the xcex/4 resonator is half as that of the xcex/2 resonator. In consequence, a total energy of the xcex/4 resonator is half as that of the xcex/2 resonator. Nevertheless, an unloaded quality factor of the xcex/4 resonator remains almost the same as that of the xcex/2 resonator since the energy loss decreases to 50% as that of the xcex/2 resonator. Accordingly, it is possible to drastically miniaturize the xcex/4 dielectric resonator without changing the resonant frequency and also the unloaded quality factor.
It is preferred that the rectangular dielectric block of the above-mentioned dielectric resonator is made of a ceramic dielectric material.
It is preferred that the resonator further includes a metal pattern partially formed on the one side surface that is different from the side surface on which the third metal layer is coated. The metal pattern may be formed on the side surface opposite to the side surface on which the third metal layer is coated, or on the side surface perpendicular to the side surface on which the third metal layer is coated.
The metal pattern has preferably a substantially rectangular shape. However, its shape is not limited to the rectangular shape, but it is possible to have an optional shape.
It is preferred that the metal pattern is an excitation electrode of the resonator. It is also preferred that the metal pattern is isolated from the first metal layer coated on the top planar surface and from the second metal layer coated on the bottom planar surface.
It is further preferred that the metal pattern has dimensions suitable for external circuit coupling.
Preferably, the resonator further includes an extension part extended from the metal pattern for control of external quality factor. This extension part is provided on the bottom planar surface.
It is preferred that the first metal layer on the top planar surface has a narrow slit for frequency tuning. It is more preferred that this slit is formed along a direction different from the direction of mode propagation.
The TEM mode dielectric resonator according to the present invention will be applied to not only a filter but also a voltage controlled oscillator (VCO) and an antenna.
According to the present invention, furthermore, a bandpass filter using a TEM mode dielectric resonator is provided. This filter includes first and second dielectric resonators each including a dielectric block having a top planar surface, a bottom planar surface, and four side surfaces, and an evanescent H-mode waveguide coupling section. Each of the first and second dielectric resonators has first and second metal layers coated on the top planar surface and the bottom planar surface, respectively, and a third metal layer coated on one of the four side surfaces. The side surface on which the third metal layer is coated is a shorted end surface and the remaining side surfaces are open to the air so that each of the first and second dielectric resonators acts as a quarter wavelength dielectric resonator and keeps an independent TEM mode of electromagnetic field. The evanescent H-mode waveguide coupling section provides TEM mode coupling between the first and second dielectric resonators by connecting the shorted end surfaces of the respective first and second dielectric resonators so as to act in an evanescent mode with a cutoff frequency higher than each resonant frequency of the first and second dielectric resonators.
As aforementioned, by using TEM dual mode half wavelength configuration in order to form a dual mode filter, dimensions of the fabricated 2 GHz filter become 8.5 mmxc3x978.5 mmxc3x971.0 mm. According to the present invention, dimensions are optimized in 3.0 mmxc3x974.25 mmxc3x971.0 mm by adopting a TEM mode xcex/4 dielectric resonator. By using two of such xcex/4 dielectric resonators, a two-pole bandpass filter is formed. Owing to this, dimensions of the filter become 3.0 mmxc3x979.0 mmxc3x971.0 mm. Thus, the volume of the bandpass filter according to the present invention becomes one-third of that of the conventional bandpass filter. Besides, the performance of the filter according to the present invention is excellent.
Two-pole and multi-pole filters each using an adequate number of xcex/4 resonators are described in the literature [3]. However, it should be noted that these filters are TE mode dielectric waveguide resonator filters.
Although such TE mode dielectric waveguide resonator filters have superior in performance, dimensions and volume in comparison with the conventional cavity filter, recent small and lightweight mobile terminals demand much miniaturized and high performance filters. Hence, in the present invention, by using TEM mode xcex/4 dielectric resonators, a two-pole bandpass filter is formed. The resonant frequency of the dominant TE mode resonator varies depending upon the change in its length and its thickness, whereas the resonant frequency of the TEM mode resonator is independent to the change in its thickness. Hence, according to the present invention, it is possible to optimize the thickness of the resonator as a function of an unloaded quality factor at a specific resonant frequency. Therefore, according to the present invention, a further miniaturized and advanced performance bandpass filter in comparison with the conventional bandpass filter can be provided.
It is preferred that the first and second dielectric resonators are made of the same dielectric material. It is more preferred that these first and second dielectric resonators are made of ceramic dielectric material with a high dielectric constant. Preferably, the evanescent mode waveguide coupling section is made of the same dielectric material with the first and second dielectric resonators.
It is also preferred that the first and second dielectric resonators have the almost same dimensions.
It is preferred that the evanescent H-mode waveguide coupling section has a shorter length and a smaller cross section than these of each of the first and second dielectric resonators. It is more preferred that dimensions of the evanescent H-mode waveguide coupling section are selected so as to obtain a desired coupling between the first and second dielectric resonators.
It is also preferred that the evanescent H-mode waveguide coupling section has a rectangular cross section.
It is preferred that the evanescent H-mode waveguide coupling section provides series coupling inductance and a pair of shunt coupling inductances between the first and second dielectric resonators.
It is preferred that the second metal layer coated on each of the bottom planar surfaces of the first and the second dielectric resonators is used as a ground plane. More preferably, the ground plane is extended to the two open side surfaces in each of the first and second dielectric resonators.
It is preferred that the side surface opposite to or perpendicular to the shorted end surface of each of the first and second dielectric resonators has an electrical input/output port. This electrical input/output port may be a metal pattern with a rectangular, square, trapezoidal or circular shape.
It is preferred that the metal pattern is isolated from the first metal layer coated on the top planar surface and from the second metal layer coated on the bottom planar surface. It is also separated from the third metal layer.
It is also preferred that the first metal layer coated on the top planar surface of at least one of the first and second dielectric resonators has a narrow slit for frequency tuning. The slit may be formed along a direction different from mode propagation direction.
According to the present invention, another bandpass filter using a TEM mode dielectric resonator is provided. This filter includes first and second dielectric resonators each including a dielectric block having a top planar surface, a bottom planar surface and four side surfaces, and an evanescent E-mode waveguide coupling section. Each of the first and second dielectric resonators has first and second metal layers coated on the top planar surface and the bottom planar surface, respectively, and a third metal layer coated on one of the four side surfaces. The side surface on which the third metal layer is coated is a shorted end surface and the remaining side surfaces are open to the air so that each of the first and second dielectric resonators acts as a quarter wavelength dielectric resonator and keeps an independent TEM mode of electromagnetic field. The evanescent E-mode waveguide coupling section provides TEM mode coupling between the first and second dielectric resonators by connecting the open side surfaces opposite to the shorted end surfaces of the respective first and second dielectric resonators so as to act in an evanescent E-mode with a cutoff frequency higher than each resonant frequency of the first and second dielectric resonators. The two resonators are coupled by the evanescent E-mode waveguide between the open side surfaces of the respective resonators.
The volume of the bandpass filter according to the present invention is one-third of that of the conventional bandpass filter. Besides, the performance of the filter according to the present invention is excellent.
It is preferred that the evanescent E-mode waveguide coupling section has a top planar surface being open to the air, four side surfaces being open to the air and a bottom planar surface on which a metal layer is coated.
It is very preferred that the bandpass filter has attenuation poles at both sides of a passband thereof. Since the bandpass filter of the present invention has unintentional attenuation poles at both sides of the passband, the frequency characteristic outside the passband can be improved. Thus, the bandpass filter can further enhance the frequency characteristic around the slope of the passband. Concretely, this bandpass filter is configured so that one of internal coupling between the first and second dielectric resonators via the evanescent E-mode waveguide coupling section is capacitive coupling and that the other one of the direct coupling is inductive coupling.
It is preferred that the first and second dielectric resonators are made of the same dielectric material. Preferably, the first and second dielectric resonators are made of ceramic dielectric material with a high dielectric constant. More preferably, the evanescent E-mode waveguide coupling section is made of the same dielectric material with the first and second dielectric resonators.
It is preferred that the first and second dielectric resonators have the almost same dimensions.
It is preferred that the evanescent E-mode waveguide coupling section has a shorter length and a smaller cross section than these of each of the first and second dielectric resonators. It is more preferred that dimensions of the evanescent E-mode waveguide coupling section are selected so as to obtain a desired coupling between the first and second dielectric resonators.
It is also preferred that the evanescent E-mode waveguide coupling section has a rectangular cross section.
It is preferred that the evanescent E-mode waveguide coupling section provides series capacitance and a pair of shunt capacitances between the first and second dielectric resonators.
It is preferred that the second metal layer coated on each of the bottom planar surfaces of the first and the second dielectric resonators is used as a ground plane. It is also preferred that the bottom planar surface on which the metal layer is coated, of the evanescent E-mode waveguide coupling section is used as a ground plane.
It is preferred that the side surface perpendicular to the shorted end surface of each of the first and second dielectric resonators is used for capacitive excitation. This excitation will be performed by an electrical input/output port formed on this side surface perpendicular to the shorted end surface of each of the first and second dielectric resonators.
Preferably, the electrical input/output port is formed by a metal pattern with a rectangular, square, trapezoidal or circular shape.
It is preferred that the metal pattern is isolated from the first metal layer coated on the top planar surface and from the second metal layer coated on the bottom planar surface.
It is also preferred that the metal pattern has dimensions selected so as to obtain a desired external circuit coupling.
It is preferred that the first metal layer on the top planar surface of at least one of the first and second dielectric resonators has a narrow slit for frequency tuning.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.